1. Field of the invention
This invention relates to a packaging material for photosensitive materials for photographic purpose.
2. Description of Prior Arts
Various types of packaging materials for photosensitive materials for photographic purpose (hereafter simply referred to as "photosensitive materials") have widely been put to practical use, and various properties are required according to their uses.
As the packaging materials for photosensitive materials, various properties are necessary such as light-shielding, gas barrier, moisture proofing, antistatic property, physical strengths such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, hot tack properties (hot-seal ability), and seal ability of contraries, bag-making aptitude, delamination resistance, slipping character and the like. Generally, it is difficult to satisfy these properties by a single film layer. Therefore, a single layer film of a high-pressure low-density polyethylene (LDPE) kneaded with carbon black or a pigment, or a composite laminated film composed of a LDPE film containing carbon black or a pigment dispersed therein and a flexible sheet such as paper, aluminum foil or cellophane, etc. has been employed. An example of the conventional laminated film is shown in FIG. 11. This film is composed of a light-shielding LDPE film layer 10a, a light-shielding metal foil layer 4a laminated on it through an adhesive layer 2, and a flexible sheet layer 3 laminated thereupon through an adhesive layer 2. Another example of the conventional film is shown in FIG. 12. This film was used for packaging color photographic printing paper, and it is composed of a light-shielding LDPE film layer 10a, a light-shielding metal foil layer 4a as a metal layer, a flexible sheet layer 3, and a light-shielding LDPE film layer 10a. They are laminated in that order, and an adhesive layer 2 is provided between each layers.
On the other hand, the present inventor has already disclosed a laminated film for photosensitive materials of which physical strength was raised by combining two uniaxially stretched films (U.S. Pat. No. 4,331,725). Another laminated film for photosensitive materials has also been disclosed. This film is composed of a foamed sheet layer and two uniaxially stretched thermoplastic resin film layers having a light-shielding property and laminated on both faces of the foamed sheet layer directly or through an adhesive layer. Thickness of the foamed sheet layer is in the range of 0.3 to 2.0 mm, and expansion ratio is 5 to 50 times. Two uniaxially stretched films are located so that their stretched axes cross each other at an angle of more than 30 degrees, and the laminated film is compressed up to 40 to 85% of theoretical total thickness. Impact puncture strength and Gelbo test strength of the compressed laminated film is large, and curling hardly occurs. This film is suitable for a weight material (U.S. Pat. No. 4,565,733).
However, each packaging material has been utilized only for limited photosensitive materials because of cost and characteristics as a packaging materials. Though the above laminated films were made in order to improve the foregoing physical properties, in the case that metallic membrane layer is introduced in order to secure gas barrier, antistatic property and moisture proofing, physical properties of these conventional films were still not enough. During packaging, the films were sometimes torn or punctured or heat sealing of the films was sometimes separated, particularly in the cases of weight products and roll films having a sharp edge. In addition, when a large amount of a light-shielding material such as carbon black was added, physical strength of the film was lowered. Then, the amount was set about 3%, and total thickness of the film was more than 70 .mu.m. As the result, the film was stiff, and working efficiency of the packaging process was badly influenced. Cost of the packaging was also expensive. In the case of the laminated film shown in FIG. 12, tear strength, impact puncture strength, Gelbo test strength, etc. were not enough. In the case of the laminated film disclosed in the specification of U.S. Pat. No. 4,331,725, since two layers of uniaxially stretched film were necessary, the laminated film was thick and expensive as the packaging material, though physical properties such as tear strength were improved. In the case of the laminated film disclosed in U.S. Pat. No. 4,565,733, the laminated film was thick and its heat sealing properties were not well in order to secure light-shielding and moisture proofing. Furthermore, since bag-making aptitude was also not well, bag for packaging photosensitive materials was formed by using a heat sealer having a special cooling device, and a hot bar was pressed through a heat-resistant film. As a result of these, the laminated film of U.S. Pat. No. 4,565,733 was expensive. Furthermore, in the cases of conventional packaging materials containing a metal foil layer, their physical strength is lowered, and thereby, they were sometimes torn or a pinhole problem encountered.
A packaging film for photosensitive materials which is a laminated film at least containing a metallized film layer and two polyethylene polymer layers which are located both sides of the metallized film layer has also been disclosed by the present inventor (U.S. patent application Ser. No. 769,573). In this packaging film, at least one of the above polyethylene polymer layers contains more than 50 wt. % of linear low density polyethylene manufactured by low pressure method, one side or both of the above polyethylene polymer layers contains 0.3 to 30 wt. % of a light-shielding material, and strength of one side or both of the above polyethylene polymer layers is stronger than adhesive force between the above metallized film and difference in the thickness of the polyethylene polymer layers is smaller than 50%.
This film is superior various physical strength, moisture proofing, gas barrier, light-shielding and antistatic character.